High power, high brightness diode lasers have been sought since the invention of the semiconductor laser. In recent years, this demand has been strongly driven by an increasing number of emerging applications, such as free space optical communication and sensing, laser material processing, medicine, display, fundamental science, and laser weapons. High power can be obtained by increasing the physical size of a diode laser, e.g., using a wider emitting aperture and longer cavity. High spatial brightness requires the single transverse mode operation to generate diffraction-limited, single-lobe far fields. Each of these approaches has limits and disadvantages.
Semiconductor diode lasers provide many unique advantages over other laser systems, such as a wide range of operation wavelengths, high electrical to optical conversion efficiency, high compactness, and low cost. Two examples of diode lasers are contained in, for example, U.S. Pat. No. 5,337,328 to Lang et al. and U.S. Pat. No. 6,122,299 to DeMars et al., both of which are incorporated by reference herein in their entirety. On the negative side, high power, high brightness (diffraction-limited beam quality) operation is difficult to obtain due to highly nonlinear materials and strong coupling between gain and index. Today, broad area diode lasers are usually used for high power applications, such as material processing and pumping sources for solid-state and fiber lasers. A few watts output power can be obtained by use of a single emitter and a few hundred watts output power can be obtained by use of a laser bar that consists an array of broad area lasers. Due to lack of transverse mode control and strong nonlinear effects, broad area lasers exhibit poor beam quality and cannot provide high brightness. Broad area laser arrays have even worse beam quality. The most common method to control the transverse mode of a diode laser is to use a step-index waveguide structure. However, for a conventional index-guided edge emitting laser, e.g., a ridge waveguide or buried waveguide laser, the laser width has to be less than a few microns in order to obtain the single transverse mode operation to maintain good beam quality. This limits the laser output power less than a few hundred miliwatts. Increasing the laser emitting width while maintaining a single transverse mode not only scales up the output power but reduces the far field beam divergence, which is highly desired for high power, high brightness applications.
There are a variety of techniques available to obtain a large aperture single transverse mode diode laser. Depending on if external optical components are needed, these techniques can be divided into two categories. The first category can be monolithically implemented, including: 1) evanescently coupled laser arrays, where multiple narrow single transverse mode lasers are coupled together to form a super transverse mode; 2) chirped and Y-coupled laser arrays; 3) leaky wave coupled (anti-guided) laser arrays; 4) unstable resonators, e.g., curved mirror or tapered lasers. These unstable resonators can be designed as either independent lasers or amplifiers in master oscillator power amplifier (MOPA) configuration; 5) grating confined broad area lasers (α-DFB), where angled gratings are used to select a single transverse mode and provide strong modal discrimination. The second category typically requires external optical components and includes externally injection locked laser arrays, external cavity laser arrays through diffractive coupling, and/or discrete MOPA lasers. By use of these mode control techniques, few watts diffraction-limited output power can be obtained. Further increasing of the optical power in a single emitter is limited by Catastrophic Optical Damage (COD) and stability/thermal problems.
Higher optical power and brightness can be obtained through laser beam combining. There are two main beam combining techniques: coherent beam combining (CBC) and spectral beam combining (SBC). SBC systems rely on an external grating to spatially overlap beams from different lasers which operate at different wavelengths. SBC is an incoherent combining technique and results in very broad spectrum width. In CBC systems, all the lasers operate at the same wavelength and are phase-locked. Previously discussed laser arrays actually can be considered as early CBC systems. CBC not only scales up the total optical power but also increases the coherent emitting aperture. Therefore, the beam divergence in CBC systems is further reduced. Currently, most CBC systems are based on the MOPA configuration with active feedback. A single frequency laser output is split and amplified, e.g., by high power fiber amplifiers. The phase of each individual amplified beam is controlled by a discrete optical phase modulator. The phase difference among the array elements is detected and then feedback to control the phase modulator in order to phase lock all the beams. Although current SBC and CBC systems can provide high diffraction-limited power, they cannot be monolithically implemented and are inefficient, complex, bulky, and expensive. Furthermore, it is very difficult to apply these techniques for diode laser arrays.
As such, an improved diode laser is desired in the art. In particular, a diode laser that provides relatively high power and high brightness would be advantageous. Further, a diode laser that does not require external optical components would be desired.